1. Field of the Invention
The present invention relates to an image processing apparatus for decoding an N-value image to an M-value image (N&lt;M).
2. Related Background Art
As a conventional technique for decoding a binary image to a multivalue image, a technique for smoothing binary data by a smoothing filter to consequently obtain multivalue data is known. For example, if filter coefficients are all "1"s, the number of dots in the filter is directly obtained as multivalue data.
There is also proposed a technique for detecting a pattern of dots of binary data using a rectangular window or a window having another shape, and switching smoothing data in accordance with the detected pattern.
However, according to the conventional multivalue image decoding technique using the smoothing filter, edge sections of characters or line images are blurred, and a completely smooth gradation expression cannot be made in a halftone section. Thus, dot patterns of a binary image often cannot be completely removed.
In the technique for switching smoothing windows for a specific pattern of a binary image, an uneasy or noisy image may be obtained in a boundary section due to an excessive density difference between a switched decoded pixel and surrounding non-switched pixels.
In either technique, it is difficult to perform perfect multivalue image decoding using the smoothing filter. In particular, binary characteristics upon conversion from a multivalue image into a binary image are not taken into consideration at all, and it is difficult to perform good multivalue image decoding.